Photocurable prepolymers and compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. In the field of photosensitive flexographic printing plates, the plates typically comprise a support and a photosensitive surface or layer prepared from a photocurable composition. Additional layers or surfaces on the plate include slip and release films to protect the photosensitive surface. Prior to processing the plate, the additional layers are removed, and the photosensitive surface is exposed to radiation in an imagewise fashion. The unexposed surfaces are then removed in developer baths.
Removal of unexposed surfaces comprising solid photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 require the use of developer baths comprising environmentally unsafe, organic solvents such as tetrachloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol. However, due to the toxicity, high volatility and low flash point of such solvents, their use gives rise to hazardous conditions and creates pollution problems. Thus, recently there has been a strong interest in the field to develop photosensitive layers in non-organic solvent developing solutions, e.g. aqueous, surfactant-aqueous or alkaline-aqueous solutions. However, the compositions resulting from recent attempts to achieve aqueous developable plates demonstrate deficiencies in mechanical properties, e.g. flexibility, softness, etc. See European Application 261,910.
For instance, in addition to possessing an aqueous developable photosensitive surface, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should possess a low hardness, i.e. sufficient softness, to facilitate ink transfer during printing. A typical hardness goal for printing plates in the field of flexible printing plates has been a Shore A hardness in the range of about 30-60.
Previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks typically used in printing. For example, European Patent Application 261,910 describes an aqueous developable printing plate prepared from a photosensitive composition comprising a ternary copolymer and a basic nitrogen containing compound such as aminoacrylates. However, the applicants of the '910 patent application illustrate in their Table that their plates have Shore A hardnesses in the range of 66-81.
Furthermore, other water-developable photosensitive compositions which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, or the like, are also insufficient in flexibility and possess a high degree of rubber hardness and hence are unsuitable for use in flexographic printing plates. Finally, it is also important that the photosensitive layer of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties when used in solid flexographic printing plates in that the compositions become tacky and pasty during storage. Those inferior properties have been attributed to the low molecular weight of the polymers used to prepare the printing plates. See U.S. Pat. No. 4,762,892 to Koch et al. and discussion of low molecular weight polymers disclosed in Japanese Kokoku 57-23693.